Four leading US technology companies have been accused of helping China to subdue political strife in return for access to its internet market.

The accusation was levelled against Microsoft, Yahoo, Cisco Systems and Google at a Congressional hearing on the ethics of doing business in China.

US Lawmakers last month accused them of giving into pressure from the Chinese government by censoring websites.

The four companies defended their co-operation with Beijing.

They conceded that restricted online access was not ideal, but argued that it was still valuable for the people of China.

China's 111-million internet population is second in size only to the US.

Critics jailed

The House International Relations subcommittee's top Democrat, Tom Lantos, told representatives of the companies that they had accumulated great wealth and power, "but apparently very little social responsibility".

"Your abhorrent actions in China are a disgrace. I simply don't understand how your corporate leadership sleeps at night," the Associated Press quoted him as saying.

Last month Mr Lantos said there had been "a string of disturbing incidents" in which US-based companies had "caved in to Beijing for the sake of profits".

Media watchdog Reporters Without Borders has accused Yahoo of providing China with information that helped to identify and convict two internet writers.

Li Zhi was jailed for eight years in 2003, after posting comments that criticised official corruption. Writer Shi Tao was sentenced to 10 years in prison in April 2005 after criticising human rights abuses.

And Google came under fire last month after it announced it would block politically sensitive terms on its new China search site, in agreement with conditions set by Beijing.

Appeal for help

The BBC's Matthew Davis in Washington says the controversy has particular resonance because it concerns the suppression of political expression on the internet, a technology that many users see as an empowering and democratising force - where it is allowed to be.

China's rapidly expanding online market of some 111 million users, 64 million of whom have broadband, has become a powerful magnet for foreign investment and for a steady stream of IT professionals from around the world.

I do think information flow is happening in China ... saying that even by existing there, contributions to a national dialogue have taken place.

Yet the Chinese government enforces strict laws on internet use, blocking content it considers a threat, including references to the Tiananmen Square massacre and notable dissidents.

In a joint statement issued last month, Microsoft and Yahoo said they lacked the leverage on their own to influence world governments.

In prepared statements the companies appealed to the hearing on Wednesday, asking for direction on how best to operate in China.

Elliot Schrage from Google said that "the requirements of doing business in China include self-censorship - something that runs counter to Google's most basic values and commitments as a company," AP reported.

Nonetheless, he said, Google had opted to enter the Chinese marketplace because it "will make a meaningful, though imperfect, contribution to the overall expansion of access to information in China".

Microsoft founder Bill Gates last month told delegates at the World Economic Forum that state censorship was no reason for technology companies not to do business in China.

Mr Gates said the internet "is contributing to Chinese political engagement" as "access to the outside world is preventing more censorship".